Although a vertical MOSFET, for example, has been conventionally used as a switching device having a high-switching speed and a large output, the gate threshold voltage tends to reduce by thinning a gate insulating film. The thinner the insulating film, even a little energy such as a static electricity is easy to insulating broken down. Therefore, a structure in which a protective diode is inserted between a gate and a source to discharge a static electricity has been conventionally used. The protective diode becomes a bidirectional zener diode by forming a p-n junction at peripheral portion of a gate electrode pad consisting of polysilicon film and is connected between the gate and the source, and FIG. 9(a) shows a sectional view of the vertical MOSFET having a structure provided with such a protective diode.
That is, an n+-type semiconductor layer 21 (epitaxial grown layer) as a drain area, is epitaxial grown on an n+-type semiconductor substrate 21a, a p-type body area 22 is formed by diffusing a p-type impurity on a surface side thereof, and an n+-type source area 23 is formed on a peripheral part of the body area 22. A gate electrode 25 is provided on a surface side of the semiconductor layer 21 positioned in an edge portion of the body area 22 and outside thereof through a gate oxide film 24. A source electrode (a source wiring) 27 is formed by Al through a contact hole provided on an interlayer insulating film 24 so as to connect with the source area 23, and a drain electrode which is not shown is formed on a back side of the semiconductor substrate 21a, thereby forming an FET portion 20.
As shown in a plan view of FIG. 9(b), the body area 22 is formed like a matrix, and a plurality of transistor cells are formed, whereby a power MOSFET corresponding to a high current is formed.
Further, in a protective diode 30 is provided in a gate electrode pad 33 consisting of a polysilicon film which is formed on a surface of a p-type area 31 formed on the n-type semiconductor layer 21 by diffusion same as the body area 22 through an insulating layer 32, and as shown in a plan explanation view of FIG. 10(a), an n-type layer 33a and a p-type layer 33b are alternately formed on a peripheral portion of the gate electrode pad 33, whereby the most peripheral n-type layer 33a having an n-p-n-p-n connecting structure is connected to the above-mentioned source electrode 27. As a result, as shown in an equivalent circuit diagram of FIG. 10(b), the protective diode 30 consisting of a bidirectilnal zener diode ZD is formed between a gate G and a source S of the FET. In FIG. 9, a numeral 35 corresponds to a gate electrode pad for bonding and a gate wiring made of a metal such as Al formed to connect with the gate electrode pad 33 consisting of a polysilicon.
As mentioned above, a conventional protective diode has a structure that the diode is provided on the peripheral portion of the gate electrode pad consisting of the polysilicon. In the structure provided on the peripheral portion of the gate electrode pad, a joint area of the protective diode (a length in a perpendicular direction for a p-n junction and a peripheral length of the gate electrode pad) cannot be enlarged sufficiently. Therefore, a withstanding voltage (breakdown voltage) can not be improved sufficiently since a series resistance of the protective diode is increased, there is a problem that the property is deteriorated due to becoming narrower a transistor cell, or a chip dimension must be enlarged, since the gate electrode pad must be enlarged.
The present invention is made to solve such problems, and an object of the present invention is to provide a semiconductor device having a protective diode that has a low series resistance and can realize its protective function sufficiently with utilizing unoccupied peripheral portion of a chip without enlarging a chip area (dimension).
Another object of the present invention is to provide a semiconductor device having a structure that can transmit signals by a wiring, having a low resistance, on the whole of transistor cells, by utilizing the protective diode provided the peripheral portion of the chip without providing a gate finger in transistor cells.
Still another object of the present invention is to provide a semiconductor device having the protective diode that can improve a breakdown strength by preventing a damage due to a partial breakdown of the protective diode or reducing a series resistance by selection of material thereof or enlarging a joint area.
Still another object of the present invention is to provide a semiconductor device having a structure that improves a withstanding voltage of the most peripheral cell of transistor cell group.